These organisms have a unique mu T cell receptor and have epipubic bones. They can be classified based on whether they are diprotodont or polyprotodont. Some families of them have paired sperm, most notably the Didelphidae family, and most of them have biphid reproductive organs. A namesake facial tumor disease is threatening the largest extant carnivore in this group of organisms. Their milk can change composition, one of the consequences of increased importance of the lactation phase in development. Extinct organisms in this group include the thylacine, variously known as a certain island's 
These organisms have a unique mu T cell receptor and have epipubic bones. They can be classified based on whether they are diprotodont or polyprotodont. Some families of them have paired sperm, most notably the Didelphidae family, and most of them have biphid reproductive organs. A namesake facial tumor disease is threatening the largest extant carnivore in this group of organisms. Their milk can change composition, one of the consequences of increased importance of the lactation phase in development. Extinct organisms in this group include the thylacine, variously known as a certain island's "tiger" or "wolf." For 10 points, name this group of mammals, most of which have their young develop in a pouch.
These organisms have a unique mu T cell receptor and have epipubic bones.
{marsupials} [prompt on {Metatheria}]